1. Field of the Invention
The present invention relates to a linear-motion helicoid mechanism applied to focusing system of a CCTV camera.
2. Description of the Related Art
Conventionally, a linear-motion helicoid mechanism is, for example, used for a focusing mechanism of a CCTV (closed circuit television) camera which may be utilized as a surveillance camera etc. A conventional linear-motion helicoid mechanism will be explained referring to FIG. 9. In a linear-motion helicoid mechanism, a lens group B can be made-to move back and forth in the direction of an optical axis PP without causing rotation of the lens frame 13 holding the lens group B. The outer peripheral surface of the cylindrically shaped lens holder 13B of the lens frame 13 is provided with a male helicoid 13A and female keyways 13b.
The female keyways 13b are for controlling the lens frame 13 movement translationally in the axial direction. The lens frame 13 is supported with a camera body (not shown), in a manner which enables it to move translationally in the direction of the optical axis PP, through a key member 15. The key member 15 is provided with keys 15a, each of which engages with each of the keyways 13b. Note that the key member 15 is fixed to a mount 16 by a mount spring (an annular spring plate) 17 and screws 18, so that the key member 15 is threadably supported by the camera body through the mount 16. The key member 15 is sandwiched between a helicoid female member 50, provided with a female helicoid 50a for engagement with the male helicoid 13a provided in the lens frame 13, and a focus ring 51. That is, the relative arrangement of the key member 15 and the helicoid female member 50 in the direction of the optical axis PP is fixed, and the distance of the helicoid female member 50 from the camera body is fixed. On the other hand, the helicoid female member 50 is able to rotate about the key member 15 concentric with the optical axis PP. The helicoid member 50 is fixed to the focus ring 51 by three screws 52 (only one shown). Namely, the helicoid member 50 can be rotated about the optical axis PP by the rotational operation of the focus ring 51. If the helicoid female member 50 is made to rotate by making the focus ring 51 rotate, the lens frame 13 is pushed out from the front to the rear or the rear to the front, by the helicoid mechanism. At this time, the lens frame 13 cannot rotate and can only move translationally along the optical axis PP as the key member 15 is fixed to the camera body.
An object of the present invention is to provide a simple and miniaturized linear-motion helicoid mechanism that may be used for a CCTV camera, at a low price.
According to the present invention, a linear-motion helicoid mechanism for a CCTV camera is provided with a lens frame, a key member, a helicoid female member, and a rotating frame that is formed integrally with the helicoid female member.
The lens frame is for holding lenses and it comprises a cylindrical lens holder having a first multicrest helicoid and at least one keyway on an outer periphery of the cylindrical lens holder. The key member comprises a cylindrical part having at least one key for engagement with the keyway at an inner periphery of the cylindrical part of the key member. The helicoid female member comprises a cylindrical part having a second multicrest helicoid that screws together with the first multicrest helicoid, at an inner periphery of the cylindrical part of the helicoid female member. The rotating frame fixes the position of the helicoid female member to the key member in an optical axis direction of the lenses and rotatably supports the helicoid female member about the optical axis;
Preferably, the rotating frame and the key member are engaged by a bayonet mechanism, whereby the position of the helicoid female member is fixed to the key member in the optical axis direction and rotatably supported about the optical axis. Due to this, assembly of the rotating frame and key member is simplified and the efficiency of the assembly process is improved.
Preferably, the bayonet mechanism is comprised of a plurality of first pawls provided at the key member and projecting out radially about the optical axis and a plurality of second pawls provided at the rotating frame and projecting in toward said optical axis to engage with each of the first pawls.
The rotating frame and the helicoid female member may be connected by an annular part perpendicular to the optical axis, and openings of the same shape as the second pawls are provided at positions of the annular part facing the second pawls. This simplifies integral molding of the rotating frame and the helicoid female member.